# Useful Speed and Feed Calculations

As a new operator, you may or may not understand the importance of speeds and feeds for different tools and materials.  In general, aluminums like higher rpms and steels enjoy slower rpms.  Remember I am speaking in very general terms here.

For example, the surface speed for 6061 AL with a carbide cutter is going to be between 1500-2400, however 304SS is going to be between 180-200.  It’s ok if you don’t understand the concept of surface speed, but you should be able to find them in the machinist handbook.  Your shop should have one lying around.

First, lets define some abbreviations:

SFM = Surface Feet per Minute

RPM = Revolutions per Minute

IPM = Inches per Minute

IPT = Inches per Tooth

DIA = Diameter (of tool)

NT = Number of Teeth ( flutes)

Now, the calculations:

SFM = RPM x DIA ÷ 3.82

This is useful if you want to figure out what surface speed the program is running.  Perhaps you want to use a bigger or smaller tool but keep the same SFM (or you are looking to win an argument with your foreman).

RPM = 3.82 x SFM ÷ DIA

Use this calculation when you know the SFM for the material/tool to get the proper RPM.  This of course may need tuned faster or slower depending on how it is cutting.

IPM = IPT x NT x RPM

Once you know what RPM you should be running, use this formula to figure out your feedrate.  Lets say we have a 5 flute 3/4 endmill to run at 970 rpm and a .005” chipload.  Lets plug these values into the formula:   IPM = .005 x 5 x 970—IPM  = 24.25IPM

So the feeedrate for that endmill should be 24.25IPM..

IPT = IPM ÷ (NT x RPM)

Sometimes when  troubleshooting a chatter problem or short tool life (or arguing with the foreman), you need to know what the chipload is on a particular cutter.  Use this formula to figure it out.

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